In facilities such as a boiler, a combustor, and the like, the state of a flame or gas that changes in accordance with the operation condition is detected, the detection results are reflected in the operation, and in addition, the degradation state of the facilities is examined to maintain the upkeep of the facilities. Since gases allow electromagnetic waves to pass therethrough more easily than solids, gases are suitable for optical monitoring. In particular, light in the near-infrared region is suitable for monitoring a gas generated from an organic substance because an absorption spectrum of a gas of a hydrocarbon or the like is located in the near-infrared region. However, in analysis by near-infrared spectroscopy, an output signal includes a large amount of noise due to a light-receiving element. Therefore, in order to extract necessary information regarding an output signal without totally depending on an improvement of the performance of sensors (light-receiving elements), a spectroscopic method, chemometrics, or the like has been used as an important method.
In the near-infrared region, the above-mentioned sensors (light-receiving elements) are broadly divided into electron tubes and photodiodes (PDs) which are solid-state components. Among these sensors, PDs have a small size and can be easily highly integrated to form a one-dimensional array, a two-dimensional array, or the like, and thus research and development of PDs has been widely performed (Non-Patent Literature (NPL) 1). The present invention targets a detection device for biological components, the detection device including a PD. Currently, the following PDs or PD arrays are used. (1) An example of such PDs or PD arrays is PDs or arrays thereof having sensitivity up to the infrared region and also having sensitivity in the near-infrared region. Specific examples of such PDs include germanium (Ge)-based PDs, lead sulfide (PbS)-based PDs, HgCdTe-based PDs, one-dimensional arrays thereof, and two-dimensional arrays thereof. (2) Another example of such PDs or PD arrays is InP-based PDs having sensitivity at a wavelength of 1.7 μm or less in the near-infrared region, InGaAs-based PDs included in the category of the InP-based PDs, and arrays thereof. Herein, the InP-based PDs refer to PDs including an absorption layer composed of a group III-V compound semiconductor and formed on an InP substrate, and InGaAs-based PDs are also included in the InP-based PDs.
Among the above photodiodes, photodiodes described in (1) are often cooled in order to reduce noise. For example, most of the photodiodes are operated while cooling at the liquid nitrogen temperature (77 K) or while cooling with a Peltier device. Accordingly, devices including such photodiodes have a large size, and the device cost is increased. Although such devices can be used at room temperature, the devices have a problem that a dark current is high in the wavelength range of 2.5 μm or less and the detection capability is poor. On the other hand, the InP-based PDs described in (2) have the following disadvantages: (I) In InGaAs, which is lattice-matched to InP, although a dark current is low, the sensitivity of the PD is limited to a wavelength range of 1.7 μm or less in the near-infrared region. (II) In extended-InGaAs, in which the wavelength region where light can be received is extended to 2.6 μm, the dark current is high, and cooling is necessary. Accordingly, in the InP-based PDs, light having a wavelength of 2.0 μm or more, which is important for improving the accuracy in gas monitoring, cannot be used or it is necessary to cool the PDs in order to use the light.
With regard to an example of optical monitoring using near-infrared light, in the maintenance of an oil-filled instrument containing insulating oil therein, such as an oil-filled (OF) cable, the degradation of the oil-filled instrument is examined by detecting the composition ratio of a plurality of hydrocarbons contained in a gas dissolved in the oil (Patent Literature (PTL) 1 and PTL 2). In particular, PTL 2 proposes a device configured to detect the concentration of a hydrocarbon gas and the concentration of hydrogen, which has no absorption spectrum in the infrared region due to the diatomic molecule thereof. In this optical monitoring device, a light-receiving element having sensitivity in a wavelength range of 1.5 to 1.6 μm is used.
As another example, in order to suppress the generation of nitrogen oxides, soot, and carbon monoxide in a combustion device such as a boiler, an optical monitoring device configured to monitor a combustion state has been proposed (PTL 3). In this device, a multilayer light-receiving element in which a silicon photodiode and a PbS photoconductive element are stacked is used. In addition, a multilayer light-receiving element including a silicon photodiode and a Ge photodiode in combination, and a multilayered light-receiving element including a silicon photodiode and a PbSe photoconductive element in combination have also been proposed. The reason why a silicon photodiode is used is to receive light having a wavelength in the visible light region or light having a wavelength near the visible light region.
Furthermore, an infrared camera that detects the temperature distribution of the entire part of an incinerator in a combustor has been proposed (PTL 4). The content of a light-receiving portion of this infrared camera is not known.
In a manufacturing process of a large-scale integrated circuit (LSI) or the like, high-purity gases are used in deposition of epitaxial films. However, these gases contain trace moisture, which adversely affects durability of the LSI or the like. To monitor such trace moisture contained in a gas, a device configured to receive transmitted light of the gas using a laser light source that oscillates at a single wavelength in the near-infrared region, and to detect a moisture concentration on the order of 0.1 to 1 ppm by lock-in detection has been proposed (NPL 2). A germanium photodiode is used as a light-receiving element in this device.
In the light-receiving devices for detecting a gas component and other general near-infrared light-receiving devices, a single element or an array of elements of InGaAs, PbS, Ge, HgCdTe, an extended-InGaAs including multistage step buffer layers, or the like is used. A light-receiving wavelength range common to all the above-mentioned gas monitoring devices is 1 to 1.8 μm. However, some of the devices determine the upper limit of the light-receiving wavelength range to about 2.0 μm or 2.5 μm.
As described above, as for InGaAs, it is necessary to extend the sensitivity to the long-wavelength side of the near-infrared region. To improve the sensitivity, the methods below have been proposed.    (K1) The indium (In) proportion of an InGaAs absorption layer is increased, and lattice mismatching between the absorption layer and an InP substrate is absorbed by interposing step buffer layers, in which the In proportion is changed stepwise, therebetween (PTL 5).    (K2) Nitrogen (N) is incorporated in an InGaAs absorption layer to form a GaInNAs absorption layer (PTL 6). Lattice matching with an InP substrate is satisfied by incorporating a large amount of N.    (K3) An extension of the light-receiving wavelength range to the long-wavelength side is realized by providing a type-II multiquantum well structure composed of GaAsSb and InGaAs (NPL 3). Lattice matching with an InP substrate is satisfied.    (K4) Formation of a two-dimensional array is realized by forming element separation trenches between light-receiving elements (pixels) by wet etching (PTL 7).    NPL 1: Masao Nakayama “Technology trend of infrared detecting elements”, Sensor Technology, 1989 March issue (Vol. 9, No. 3), p. 61-64    NPL 2: Shang-Qian Wu, et al., “Detection of trace moisture in gas with diode laser absorption spectroscopy”, Journal of the Japan Society of Infrared Science and Technology, Vol. 11, p. 33-40 (2001)    NPL 3: R. Sidhu, “A Long-Wavelength Photodiode on InP Using Lattice-Matched GaInAs—GaAsSb Type-II Quantum Wells”, IEEE Photonics Technology Letters, Vol. 17, No. 12 (2005), pp. 2715-2717    PTL 1: Japanese Unexamined Patent Application Publication No. 2007-120971    PTL 2: Japanese Unexamined Patent Application Publication No. 09-304274    PTL 3: Japanese Unexamined Patent Application Publication No. 05-79624    PTL 4: Japanese Unexamined Patent Application Publication No. 05-196220    PTL 5: Japanese Unexamined Patent Application Publication No. 2002-373999    PTL 6: Japanese Unexamined Patent Application Publication No. 9-219563    PTL 7: Japanese Unexamined Patent Application Publication No. 2001-144278